Feed-forward filtering device and associated method

ABSTRACT

A filtering device includes a low-pass filter (LPF), a noise estimation circuit and a first combining circuit. The LPF receives and filters a pre-filtering signal to generate an output signal of the filtering device. The noise estimation circuit estimates an estimated noise signal according to the output signal and the pre-filtering signal. The first combining circuit subtracts the estimated noise signal from an input signal of the filtering device to generate the pre-filtering signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/301,631, filed on Mar. 1, 2016 and incorporated herein by reference.

BACKGROUND

In an integrated circuit (IC) design, circuitry comprising hundreds ofcircuit blocks has to be designed within a limited area. AResistor-Capacitor (RC) low-pass filter is the most common filter usedfor filtering out undesired noise, but resistors cost a larger area inmore advanced IC technology. Therefore, a filtering technique which canreduce the area of the filter for an RC based filter while also beingable to effectively filter out undesired noise is required.

SUMMARY

One of the objectives of the present invention is to provide a filteringdevice and an associated filtering method to solve the abovementionedproblems.

According to a first aspect of the present invention, a filtering deviceis disclosed, wherein the filtering device comprises a low pass-filter(LPF), a noise estimation circuit, and a first combining circuit. Thelow pass-filter is arranged to receive and filter a pre-filtering signalto generate an output signal of the filtering device; the noiseestimation circuit is arranged to estimate an estimated noise signalaccording to the output signal and the pre-filtering signal; and a firstcombining circuit is arranged to subtract the estimated noise signalfrom an input signal of the filtering device to generate thepre-filtering signal.

According to a second aspect of the present invention, a filteringdevice is disclosed, comprising a low-pass filter (LPF), a feedbackcircuit and a resistor circuit. The low-pass filter is arranged toreceive and filter a pre-filtering signal from an interconnection nodeto generate an output signal of the filtering device; the feedbackcircuit is arranged to receive the output signal to generate a feedbacksignal; and the resistor circuit is coupled between an input signal ofthe filtering device and the feedback signal to generate thepre-filtering signal at the interconnection node; wherein the inputsignal, the pre-filtering signal and the output signal are voltagesignals.

According to a third aspect of the present invention, a filtering methodis disclosed, wherein the filtering method comprises: performing alow-pass filtering (LPF) operation upon a pre-filtering signal togenerate an output signal; estimating a estimated noise signal accordingto the output signal and the pre-filtering signal; and subtracting theestimated noise signal from an input signal to generate thepre-filtering signal.

According to a fourth aspect of the present invention, a filteringmethod is disclosed, wherein the filtering method comprises: performinga low-pass filtering (LPF) operation upon a pre-filtering signal from aninterconnection node to generate an output signal; generating a feedbacksignal according to the output signal; and utilizing a resistor circuitcoupled between an input signal and the feedback signal to generate thepre-filtering signal at the interconnection node; wherein the inputsignal, the pre-filtering signal and the output signal are voltagesignals.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a filtering device according to anembodiment of the present invention.

FIG. 2 is a diagram illustrating a noise estimation circuit of thefiltering device of FIG. 1 according to an embodiment of the presentinvention.

FIG. 3 is a circuit diagram of a filtering device implemented accordingto the filtering devices shown in FIG. 1 and FIG. 2.

FIG. 4 is a diagram illustrating a simplified circuit of the filteringdevice shown in FIG. 2.

FIG. 5 is a diagram illustrating a simplified circuit of the filteringdevice shown in FIG. 3.

FIG. 6 is a diagram illustrating a simplified circuit of the filteringdevice shown in FIG. 4.

FIG. 7 is a diagram illustrating a filtering device implementedaccording to the filtering device shown in FIG. 6.

DETAILED DESCRIPTION

Certain terms are used throughout the description and following claimsto refer to particular components. As one skilled in the art willappreciate, manufacturers may refer to a component by different names.This document does not intend to distinguish between components thatdiffer in name but not function. In the following description and in theclaims, the terms “include” and “comprise” are used in an open-endedfashion, and thus should not be interpreted as a close-ended term suchas “consist of”. Also, the term “couple” is intended to mean either anindirect or direct electrical connection. Accordingly, if one device iscoupled to another device, that connection may be through a directelectrical connection, or through an indirect electrical connection viaother devices and connections.

FIG. 1 is a diagram illustrating a filtering device 100 according to anembodiment of the present invention. The filtering device 100 may be afeed-forward filtering device. As shown in FIG. 1, the filtering device100 comprises a low-pass filter (LPF) 101, a noise estimation circuit102 and a combining circuit 103, wherein the low-pass filter 101receives and filters a pre-filtering signal S_(pre) to generate anoutput signal S_(out), the noise estimation circuit 102 generates anestimated noise signal S_(noise) according to the output signal S_(out)and the pre-filtering signal S_(pre), and the combining circuit 103subtracts the estimated noise signal S_(noise) from an input signalS_(in) to generate the pre-filtering signal S_(pre). It should be notedthat the pre-filtering signal S_(pre), the output signal S_(out) and theinput signal S_(in) illustrated here can be referred to as voltagesignals (e.g. V_(pre), V_(out), and V_(in)) or current signals (e.g.I_(pre), I_(out), and I_(in)). The noise can be greatly reduced bysubtracting the estimated noise signal S_(noise) generated by the noiseestimation circuit 102 from the input signal S_(in) when the inputsignal S_(in) enters the circuit.

FIG. 2 is a diagram illustrating the noise estimation circuit 102 of thefiltering device 100. As shown in FIG. 2, the noise estimation circuit102 may be implemented using amplifying circuits 201 and 202, and acombining circuit 203. The gain of the amplifying circuit 201 is β1,while the gain of the amplifying circuit 202 is β2. The amplifyingcircuit 201 is arranged to amplify the output signal S_(out) to generatean amplified output signal S_(amp.out), where S_(amp.out)=β1·S_(out).The amplifying circuit 202 is arranged to amplify the pre-filteringsignal S_(pre) to generate an amplified pre-filtering signalS_(amp.pre), where S_(amp.pre)=β2·S_(pre). The combining circuit 203subtracts the amplified output signal S_(amp.out) from the amplifiedpre-filtering signal S_(amp.pre) to generate the estimated noise signalS_(noise) as shown in FIG. 2. It should be noted that the configurationof the noise estimation circuit 102 shown in FIG. 2 is for illustrativepurposes only. In one embodiment, both the amplifying circuits 201 and202 can be replaced with a unity-gain buffer. In another embodiment, theamplifying circuits 201 and 202 can be optional. For example, only theamplifying circuit 201 is used and the pre-filtering signal S_(pre) isdirectly fed into the combining circuit 203, and the combining circuit203 subtracts the amplified output signal S_(amp.out) from thepre-filtering signal S_(pre), where the pre-filtering signal S_(pre) isreceived by the combining circuit 203. For another example, only theamplifying circuits 202 is used and the output signal S_(out) isdirectly fed into the combining circuit 203, and the combining circuit203 can subtract the output signal S_(out) from the amplifiedpre-filtering signal S_(amp.pre), where the output signal S_(out) isreceived by the combining circuit 203. For a further example, neitherthe amplifying circuit 201 nor the amplifying circuit 202 is used. Thecombining circuit 203 subtractS the output signal S_(out) from thepre-filtering signal S_(pre), where the output signal S_(out) and thepre-filtering signal S_(pre) are received by the combining circuit 203.

After subtracting the output signal S_(out) (or the amplified outputsignal S_(amp.out)) which is filtered by the low-pass filter 101 fromthe pre-filtering signal S_(pre) (or the amplified pre-filtering signalS_(amp.pre)) which has not been filtered, the estimated noise signalS_(noise) can be extracted.

FIG. 3 is a circuit diagram of a filtering device 300 implementedaccording to the filtering devices shown in FIG. 1 and FIG. 2. In theembodiment of FIG. 3, the pre-filtering signal S_(pre), the outputsignal S_(out), the estimated noise signal S_(noise) and the inputsignal S_(in) are referred to as current signals, i.e. an pre-filteringsignal I_(pre), an output signal I_(out), an estimated noise signalI_(noise) and an input signal I_(in) as marked in FIG. 3. As shown inFIG. 3, the filtering device 300 is composed of a plurality of currentmirrors including transistors MP1, MP2, MP3, MP4, MN1 and MN2, whereMP1, MP2, MP3 and MP4 are P-channel metal oxide semiconductor (PMOS)transistors, and MN1 and MN2 are N-channel metal oxide semiconductor(NMOS) transistors. The filtering device 300 comprises a low-pass filter301, a noise estimation circuit 302 and a combining circuit 303, whereinthe low-pass filter 301 is a passive Resistor-Capacitor filter composedof a resistor R1 and a capacitor C1, the noise estimation circuit 302 iscomposed of the transistor MP3, the transistor MN1 and a combiningcircuit 302_1 implemented by a node N2 for draining and receivingcurrent, and the combining circuit 303 is implemented by a node N1 inthis embodiment. As mentioned in the embodiment of FIG. 2, thetransistor MP3 included in the noise estimation circuit 302 is referredto as the amplifying circuit 202 for amplifying a current signal I_(pre)generated by the transistor MP2 according to its channel width orlength. The channel width of the transistor MP3 may be configured to beβ2 times as large as the channel width of the transistor MP2. In thisway, the current generated by the transistor MP3 is β2*I_(pre) when thecurrent generated by the transistor MP2 is I_(pre). The low-pass filter301 is coupled between the transistor MP1 and MP2. The pre-filteringsignal I_(pre) is received by the low-pass filter 301 via a node N3. Thelow-pass filter 301 filters the pre-filtering signal I_(pre) to generatea filtered signal at a node N4. The transistor MP1 receives the filteredsignal to generate the output signal I_(out). With the function of acurrent mirror, the transistor MP4 replicates the current of the outputsignal I_(out) and also produces the output signal I_(out). The outputsignal I_(out) flows via the drain terminals of the transistor MP4 andMN2, where in one embodiment, the channel width of the transistor MP4 issame as that of the transistor MP1 to mirror the current signal I_(out)directly on the drain terminal of the transistor MP4. By configuring thechannel width of the transistor MN1 to be β1 times as large as that ofthe transistor MN2, i.e. when the channel width of the transistor MN2 ismarked as 1, the channel width of the transistor MN1 is marked as β1, acurrent β1*I_(out) is generated from a drain terminal of the transistorMN1. Please note that the channel width of the transistor MP4 and thatof the transistor MN2, as well as the gain provided by the transistorMP4 and the gain provided by the transistor MN2, should be subject tocircuit designs and thus can be varying accordingly. In accordance withthe Kirchhoff Circuit Laws, an estimated noise signal I_(noise) isdrained from the node N2 included in the combining circuit 203_1 to thenode N1 included in the combining circuit 303, and a current signalI_(in) is thus drained from the node N1 according to the estimated noisesignal I_(noise) and the pre-filtering I_(pre).

The following current equations which illustrate the Kirchhoff CircuitLaws can explain the embodiment of FIG. 3 clearly.Iin(s)−Inoise(s)=Ipre(s)  (1)Inoise(s)=β2*Ipre(s)−β1*Iout(s)  (2)Iout(s)=H(s)*Ipre(s)  (3)wherein H(s) is the transfer function of the low-pass filter 301.

According to the equations (1)-(3) illustrated above, the relationbetween the output signal I_(out) and the input signal I_(in) can bewritten as follows:

$\begin{matrix}{\frac{{Iout}(s)}{{Iin}(s)} = \frac{1}{( {1 + {\Delta\beta}} ) + \frac{s}{\frac{\omega\; p}{1 + {\beta\; 2}}}}} & (4)\end{matrix}$wherein Δβ=β2−β1. According to equation (4), the −3 dB frequencyω_(−3dB) of the filtering device 300 is:

$\begin{matrix}{\omega_{{- 3}{dB}} = {\frac{1 + {\Delta\;\beta}}{1 + {\beta\; 2}}\omega_{p}}} & (5)\end{matrix}$

According to equation (5), the −3 dB frequency of the filtering device300 can be reduced by 1+β2 when β1=β2, which can decrease the noiseeffectively. It should be noted that, in this embodiment, Δβ must begreater than −1, or a positive feedback might occur in the filteringdevice 300.

FIG. 4 is a diagram illustrating a filtering device 400, which is asimplified circuit of that shown in FIG. 2. Referring to FIG. 2 again,by combining the combining circuits 103 and 203, the filtering device100 can be simplified accordingly. As shown in FIG. 4, the filteringdevice 400 comprises a low-pass filter (LPF) 401, a combining circuit402, and amplifying circuits 403 and 404, wherein the gain of theamplifying circuit 403 is 1/(1+β2), while the gain of the amplifyingcircuit 404 is β1. Based on the same theory, the filtering device 300can be thus simplified by combining the combining circuits 303 and 302_1shown in FIG. 3.

FIG. 5 is a diagram illustrating a filtering device 500, which is asimplified circuit of that shown in FIG. 3. As shown in FIG. 5, thefiltering device 500 is composed of a plurality of current mirrorsincluding transistors MP1′, MP2′, MP3′, MN1′ and MN2′, where MP1′, MP2′and MP3′ are PMOS transistors, and MN1′ and MN2′ are NMOS transistors.The difference between the embodiments of FIG. 3 and FIG. 5 is that thetwo transistors MP2 and MP3 shown in FIG. 3 are combined as a singletransistor MP2′ based on the concept of combining two circuits as shownin FIG. 4. After combining the transistor MP2 and MP3 as the transistorMP2′, the filtering device 500 thus comprises a combining circuit 501implemented by a node N5 whose function is similar/identical with thecombining circuit 401 for combining the current I_(in) and β1*I_(out).The filtering device comprises a amplifying circuit 502 implemented bythe transistor MN1′ whose function is similar/identical with theamplifying circuit 404 for amplifying the output current I_(out) by β1.As descried in the embodiment of FIG. 3, the low-pass filter 301generates a filtered signal at the node N4. The transistor MP1′ receivesthe filtered signal to generate the output signal I_(out). With thefunction of a current mirror, the transistor MP3′ replicates the currentof the output signal I_(out) and also produces the output signalI_(out). The output signal I_(out) flows via the drain terminals of thetransistors MP3′ and MN2′. Those skilled in the art should readilyunderstand the detailed operation of the circuit shown in FIG. 5 afterreading the above paragraphs. A detailed description is thereforeomitted here for brevity.

The embodiments of FIG. 3 and FIG. 5 are implemented by a plurality ofcurrent mirrors; however, both implementations are hard to be applied tofiltering voltage signals. In practice, a condition β1=β2 is used tosimplify the filtering device 400 for the situation where the inputsignals S_(in), the pre-filtering signal S_(pre) and the output signalS_(out) are voltage signals instead of current signals.

Under the condition β1=β2, the filtering device 400 shown in FIG. 4 canbe simplified to the circuit 600 shown in FIG. 6. The filtering device600 comprises a low-pass filter (LPF) 601, a feedback path 602,combining circuits 603 and 604, and an amplifying circuit 605, whereinthe low-pass filter 601 is arranged to receive and filter thepre-filtering S_(pre) to generate the output signal S_(out) which issent to the combining circuits 603 and 604 through the feedback path602. The combining circuit 603 is arranged to subtract the output signalS_(out) from the input signal S_(in) to generate a signal S_(x). Theamplifying circuit 605 is arranged to amplify the signal S_(x) togenerate a signal S_(y), and the combining circuit 604 is arranged tocombine the signal S_(y) and the output signal S_(out) to generate thepre-filtering signal S_(pre). It should be noted that the input signalS_(in), the signals S_(x) and S_(y), the pre-filtering signal S_(pre)and the output signal S_(out) are referred to as voltage signals in thisembodiment. The following equation (6) describes the relation betweenthe pre-filtering signal S_(pre) and the output signal S_(out) accordingto the embodiment in FIG. 6.

$\begin{matrix}{{Spre} = {{Sout}*( {\frac{- 1}{{\beta\; 2} + 1} + 1} )}} & (6)\end{matrix}$

According to equation (6), a filtering device arranged for filteringvoltage signals, instead of current signals, can be easily implemented.FIG. 7 is a diagram illustrating a filtering device 700 implementedaccording to the filtering device shown in FIG. 6. In the embodiment ofFIG. 7, the pre-filtering signal S_(pre), the output signal S_(out), andthe input signal S_(in) are referred to as voltage signals, i.e. thepre-filtering signal V_(pre), the output signal V_(out), and the inputsignal V_(in) as marked in FIG. 7. The filtering device 700 comprises alow-pass filter 701, a feedback circuit 702 and a resistor circuit 703,wherein the low-pass filter 701 is a passive Resistor-Capacitor filterincluding a resistor R2 and a capacitor C2, the feedback circuit 702comprises a feedback buffer 712, and the resistor circuit 703 comprisesa plurality of resistors Rx and Ry. More specifically, the low-passfilter 701 is arranged to receive and filter the pre-filtering signalV_(pre) to generate an output signal V_(out). The feedback buffer 712included in the feedback circuit 702 is arranged to send the outputsignal V_(out) to the resistor circuit 703. The resistors Rx and Ry areconnected in series and coupled between the output signal V_(out) and aninput signal V_(in) is arranged to serve as a voltage divider thatdivides the voltage to generate the pre-filtering signal V_(pre) shownin FIG. 7. The following equations describe the relation between theinput signal V_(in) and the output signal V_(out) according to theembodiment in FIG. 7.

$\begin{matrix}{{\lbrack {{( {{{Vin}(s)} - {{Vout}(s)}} )*\frac{Ry}{{Rx} + {Ry}}} + {{Vout}(s)}} \rbrack*{H(s)}} = {{Vout}(s)}} & (7) \\{{H(s)} = \frac{1}{1 + \frac{s}{\omega_{p}}}} & (8)\end{matrix}$wherein H(s) is the transfer function of the low-pass filter 701.

According to equations (7) and (8), it can be deduced that

$\begin{matrix}{{\frac{{Vout}(s)}{{Vin}(s)} = \frac{1}{1 + \frac{s}{\frac{\omega_{p}}{1 + {\beta\; 2}}}}},{{\beta\; 2} = \frac{Rx}{Ry}}} & (9)\end{matrix}$

Based on equation (9), the −3 dB frequency of the filtering device 700can be reduced by 1/(1+β2) to decrease the noise effectively.

In practice, the input signal V_(in) might not have enough drivingability; the filtering device 700 can therefore be configured to furthercomprise a buffer 704 coupled between the input signal V_(in) and theresistor Rx, which increases the driving ability of input signal V_(in),as shown in FIG. 7.

It should be noted that, in the embodiments of FIG. 3, FIG. 5 and FIG.7, the low-pass filters 301, 501, and 701 are implemented by passiveResistor-Capacitor filters. This is only for illustrative purposes andnot a limitation of the present invention. In other embodiments, thelow-pass filter can be implemented by other circuits such as an activelow-pass filter.

Briefly summarized, embodiments of the present invention disclosefiltering devices employing the feed forward technique to subtract noisefrom the input signal when the input signal enters the circuit. In thisway, noise can be effectively decreased without increasing theresistance of a resistor included in a low-pass filter, and the consumedarea for a low-pass filter in an integrated circuit can also beeffectively limited.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A filtering device, comprising: a low-pass filter(LPF), arranged to receive and filter a pre-filtering signal to generatean output signal of the filtering device; a noise estimation circuit,arranged to generate an estimated noise signal according to the outputsignal and the pre-filtering signal; and a first combining circuit,arranged to subtract the estimated noise signal from an input signal ofthe filtering device to generate the pre-filtering signal; wherein thenoise estimation circuit comprises: a first amplifying circuit, arrangedto amplify the output signal by a first gain to generate an amplifiedoutput signal; a second amplifying circuit, arranged to amplify thepre-filtering signal by a second gain to generate an amplifiedpre-filtering signal; and a second combining circuit, arranged tosubtract the amplified output signal from the amplified pre-filteringsignal to generate the estimated noise signal.
 2. The filtering deviceof claim 1, wherein the second gain minus the first gain is greater thanminus one.
 3. The filtering device of claim 1, wherein the input signal,the pre-filtering signal and the output signal are current signals. 4.The filtering device of claim 3, wherein the first combining circuitcomprises: an interconnection node, arranged to receive the estimatednoise signal and the pre-filtering signal, wherein the input signal isdrained from the interconnection node.
 5. The filtering device of claim3, wherein the second combining circuit comprises: an interconnectionnode, arranged to receive the amplified pre-filtering signal, whereinthe amplified output signal and the estimated noise signal are drainedfrom the interconnection node.
 6. The filtering device of claim 1,wherein the input signal, the pre-filtering signal and the output signalare voltage signals.
 7. A filtering method, comprising: performing alow-pass filtering (LPF) operation upon a pre-filtering signal togenerate an output signal; estimating an estimated noise signalaccording to the output signal and the pre-filtering signal; andsubtracting the estimated noise signal from an input signal to generatethe pre-filtering signal; wherein estimating the estimated noise signalaccording to the output signal and the pre-filtering signal comprises:amplifying the output signal by a first gain to generate an amplifiedoutput signal; amplifying the pre-filtering signal by a second gain togenerate an amplified pre-filtering signal; and subtracting theamplified output signal from the amplified pre-filtering signal togenerate the estimated noise signal.
 8. The filtering method of claim 7,wherein the second gain minus the first gain is greater than minus one.9. The filtering method of claim 7, wherein the input signal, thepre-filtering signal and the output signal are current signals.